• What role does the alignment of the Moon and Sun have on the tides?

When the sea level is above normal, it is called high tide. The opposite is low tide.

The moon’s gravity is the primary cause of the rising tide.

The Sun’s gravity also matters – but less than that of the moon

The force from the Moon pulls the ocean toward it, a maximum of about 1 m (one meter) – but it also causes water to pile up on the opposite side of the Earth?! We’ll have to figure out why that happens!

Small changes in sea-level can make big changes in shoreline rise

Where the shore is shallow, a 1 m change in sea level can result in even a 10 m or more rise along the shoreline.

Since the tides are caused by the Moon’s gravity pulling on Earth’s oceans, you might think the shape of the oceans would only be pulled toward the Moon.

Tides on opposite sides of the Earth?

The oceans are stretched so that there are high tides on both sides of the Earth. How does that happen ?!

What causes this to happen?

All parts of the Earth’s ocean are attracted toward the Moon – but the amount of pull isn’t the same everywhere.
The water closest to the moon is pulled more.
The water further away from the moon is pulled less.
So there is a difference in the amount of pull between one side of the earth and the other:
This is a gravitational differential.

Allow each of these five parts to “fall” a small distance towards the moon (due to its gravity)

It falls, or stretches, into the blue ellipse: Each side is a tidal bulge!

There is one tidal bulge on each side of the Earth.
If we subtract the motion of the earth towards the moon, and just consider how this affects the shape of the earth, then we get the above image.
The arrows indicate the strength of the force.

The Bay of Fundy

The Bay of Fundy is a bay on the Atlantic coast, on the northeast end of the Gulf of Maine, between the Canadian provinces of New Brunswick and Nova Scotia. It has highest tidal range in the world.

More than 2 tides a day?

Although the sun, the moon and the rotation of the earth are the major forces involved in creating the tides the local conditions such as the shoreline and the contour of the ocean floor also have an effect.

Because of this not everywhere has 2 tides a day – there are some places that experience what is known as a double-high water (e.g. Southampton) or double-low water (e.g. Portland).

The highest tides of all (17m) occur in Canada and after a long running dispute between the famous tides of the Bay of Fundy and those of Ungava Bay on the northern coast of Quebec, the Canadian Hydrographic Service has declared a draw.

Learning Standards

8.MS-ESS1-2. Explain the role of gravity in ocean tides, the orbital motions of planets, their moons, and asteroids in the solar system.

HS-ESS1-4. Use Kepler’s laws to predict the motion of orbiting objects in the solar system.
Describe how orbits may change due to the gravitational effects from, or collisions
with, other objects in the solar system.

HS-PS2-4. Use mathematical representations of Newton’s law of gravitation and Coulomb’s law to both qualitatively and quantitatively describe and pre

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one
dimension in various situations.

Gravitational, electric, and magnetic forces between a pair of objects do not require that they be in contact. These forces are explained by force fields that contain energy and can transfer energy through space. These fields can be mapped by their effect on a test object (mass, charge, or magnet, respectively). Objects with mass are sources of gravitational fields and are affected by the gravitational fields of all other objects with mass. Gravitational forces are always attractive. For two human-scale objects, these forces are too small to observe without sensitive instrumentation. Gravitational interactions are non-negligible, however, when very massive objects are involved. Thus the gravitational force due to Earth, acting on an object near Earth’s surface, pulls that object toward the planet’s center. Newton’s law of universal gravitation provides the mathematical model to describe and predict the effects of gravitational forces between distant objects. These long-range gravitational interactions govern the evolution and maintenance of large-scale structures in the universe (e.g., the solar system, galaxies) and the patterns of motion within them… Newton’s law of universal gravitation and Coulomb’s law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects.